The present invention relates to a mixer device for processing a radio signal. Such a device can in particular be used in the first receiving stage of a radio communication receiver. The radio signal processed can be a phase shift keying (PSK) or frequency shift keying (FSK) signal (FSK modulations may be regarded as particular cases of PSK modulations) .
The mixer device serves to extract at an intermediate frequency f.sub.I a modulated phase exhibited by the radio signal received at a communication frequency f.sub.C. To do this, use is made of a transposition frequency f.sub.O delivered by a local oscillator, equal to the sum or the difference between the communication frequency f.sub.C and the intermediate frequency f.sub.I. However, the radio signal picked up generally has a wide spectrum, and possesses a priori components at the frequency f.sub.O +f.sub.I and at the frequency f.sub.O -f.sub.I. One of these two frequencies is the desired communication frequency f.sub.C, whilst the other is an image frequency 2f.sub.O -f.sub.C which the device must reject in order for its phase not to disturb the modulated phase at the frequency f.sub.C which it is sought to extract.
FIG. 1 shows a conventional arrangement of an image frequency rejection device. A local oscillator 10 delivers two quadrature waves at the transposition frequency f.sub.O =.omega..sub.I /2.pi., which are each mixed with the input signal X by respective mixers 12, 14. With regard to the intermediate frequency f.sub.I =.omega..sub.I /2.pi., the radio signal X possesses two components to be considered, one written sin(.omega..sub.O -.omega..sub.I)t+.phi.! and the other written sin(.omega..sub.O +.omega..sub.I)t+.phi.'!. It is assumed here that the desired communication frequency f.sub.C is (.omega..sub.O -.omega..sub.I)/2.pi. and that the frequency (.omega..sub.O +.omega..sub.I /2.pi. is the undesirable image frequency. The modulated phase which it is sought to extract is then the phase .phi.. The output signal X1 from the mixer 12, resulting from the mixing of the radio signal X with the wave 2 cos .omega..sub.O t possesses two components at the intermediate frequency f.sub.I : one having the phase -.phi.+180.degree. and the other having the phase .phi.'. The output signal X2 from the mixer 14, resulting from the mixing of the radio signal X with the wave 2 sin .omega..sub.O t which has a phase lag of 90.degree. with respect to the wave 2 cos .omega..sub.O t, likewise possesses two components at the intermediate frequency: one having the phase -.phi.+90.degree. and the other having the phase .phi.'+90.degree.. A phase-shifter filter 16 applies a 90.degree. phase lag at the intermediate frequency to the signal X2 so as to produce a signal X2' having, at the intermediate frequency, a component of phase -.phi. and a component of phase .phi.'. A subtractor 18 deducts the signal X1 from this phase-shifted signal X2'. The undesirable phase .phi.' is thus eliminated from the output signal Y from the subtractor 18. Only the desired phase .phi. remains (bearing a minus sign in this particular case). The output signal Y is applied to a band-pass filter 20 which lets through the components with frequency close to the intermediate frequency f.sub.I so as to extract the desired phase.
The major drawback of the device represented in FIG. 1 is the need for a 90.degree. phase-shifter requiring two poles and generally having to be embodied in the form of discrete components. Furthermore, the paths followed by the two signals combined by the subtractor 18 are not identical, this affecting the effectiveness of the rejection.
In order to reduce the impact of these problems, it has been proposed to replace the 90.degree. phase-shifter with two .+-.45.degree. phase-shifters. FIG. 2 shows such an arrangement. The phase-shifter 15 applies a 45.degree. phase lead to the signal X1 so as to produce a signal X1" having, at the intermediate frequency, a component of phase -.phi.+225.degree. and a component of phase .phi.'+45.degree.. The phase-shifter 17 applies a 45.degree. phase lag to the signal X2 to produce a signal X2" having, at the intermediate frequency, a component of phase -.phi.+45.degree. and a component of phase .phi.'+45.degree.. The output signal Y=X2"-X1" from the subtractor 18 then possesses at the intermediate frequency a single component of phase -.phi.+45.degree., that is to say containing the useful information. It is noted that the amplitude of this signal Y is smaller than that obtained with the layout of FIG. 1, given the .sqroot.2 attenuation afforded by the .+-.45.degree. phase-shifters 15, 17.
The device of FIG. 2 improves on that of FIG. 1, especially in that it can be embodied in the form of an integrated circuit. However, it is not free of drawbacks. In particular, the construction of the two RC networks making up the phase-shifter filters 15, 17 is tricky. The matching of these two networks is tricky and requires compensations. In general, one of the capacitances is not floating and it is therefore necessary to take account of its stray capacitance, for example by placing follower stages with low output impedance upstream of the phase-shifter filters. To obtain sufficiently effective rejection, the phase-shifter networks usually have to be tuned by means of digital/analog converters. Furthermore, the device has constantly to adapt to changes in the environment (temperature, supply voltage, etc.) so as to optimize image rejection.
An object of the present invention is to propose a simplified structure for a mixer device with image frequency rejection which can be embodied entirely in the form of an integrated circuit.